1. Field of the Invention
The present invention relates to superconducting magnetic sensors and, more particularly, to a superconducting magnetic sensor utilizing magnetoresistance effect developing from a grain boundary in a superconductive ceramic material.
2. Description of the Prior Art
Hitherto, for the purpose of magnetic field detection, Hall elements utilizing the Hall effect of a semiconductor or magnetoresistors utilizing the magnetoresistance effect of a semiconductor or magnetic material have been widely used. Also, superconducting quantum interference devices (SQUID's) which utilize the quantum interference effect of a superconductor have been used as very high sensitive magnetic sensors. However, the magnetic sensitivity of such Hall elements, or such semiconductor or magnetic material magnetoresistors is of the order of 10.sup.-3 to 10.sup.-4 gauss. In order for them to be used in a wider range of applications, it is necessary that they must have a higher magnetic sensitivity. It is known that SQUID's have a higher degree of magnetic sensitivity, or a sensitivity level of 10.sup.-10 gauss, but the problem is that they require very strict configurational precision and a very complicated drive system, thus involving considerable difficulty in operational control. For these reasons, there has been much need for an improved magnetic sensor which has a high magnetic sensitivity and is easy to use.
In view of such situation, about four years ago the present inventors et al. proposed a high-performance magnetic sensor (superconducting magnetoresistor) utilizing magnetoresistance effect developing from a grain boundary in a superconductive ceramic material (U.S. Pat. No. 5,011,818). This superconducting magnetoresistor comprises a ceramic superconductive film of a specified pattern configuration placed on a substrate. It has various good characteristic features: (1) that it exhibits high magnetic sensitivity of 10.sup.-6 gauss level or a sensitivity level which is far much greater than that of any conventional magnetic sensor using a semiconductor or magnetic material (Sharp Technical Bulletin No. 45 (1990)); (2) that it is simple in construction and affords ease of handling; and (3) that it can detect analog and digital signals.
However, in order that a magnetic sensor may be employed in detecting biomagnetism, such as magnetocardiowave or pulmomagnetofield, or conducting non-destructive inspections of metal materials for corrosion or defect detection, or for resource exploration purposes, it is necessary that the sensor must have a magnetic sensitivity level of 10.sup.-7. As such, further one-figure improvement in sensitivity has been desired of the above mentioned prior-art superconducting magnetic sensor.
In order to achieve a magnetic sensitivity level of 10.sup.-7 gauss, the present inventor et al. recently proposed an arrangement wherein superconductive magnetoresistors are laminated on a substrate and electrically interconnected to provide a larger device length, so that improved magnetic sensitivity can be obtained without any decrease in space resolving capability. In this arrangement, however, the laminated superconductive magnetoresistors are electrically interconnected, and this requires a complicated process of fabrication.